1. Field of the Invention
The present invention relates to an optical module applicable to an optical receiver that realizes a digital coherent optical transmission scheme.
2. Description of the Related Art
Recently, a variety of optical communication devices are being developed in preparation for the realization of 100 Gbit/s and greater ultra high-speed optical transmission systems in order to achieve greater communication capacity. Optical communication devices utilizing digital coherent technology are attracting attention as optical communication devices used in 100 Gbit/s and greater ultra high-speed optical transmission systems. With digital coherent optical transmission schemes, in order to transmit and receive transmission data using optical interference, signal light is combined with local oscillator (LO) light to demodulate the signal when receiving, and the result is then subjected to digital signal processing. This technique compensates for signal degradation such as chromatic dispersion and polarization mode dispersion due to transmission. Given this background, the development of optical front-end modules for digital coherent receivers is being advanced.
FIG. 1 illustrates an exterior view of an optical front-end module for a digital coherent receiver (also simply called a front-end module or optical module). As illustrated in FIG. 1, the front-end module 100 is provided with two optical inputs (optical fibers) 101 and 102 that respectively input signal light and local light, terminals 106a and 106b that supply power or the like to the module inside the case 104 of the front-end module 100, and output terminals 105 which output the output signals generated by processing the two optical inputs. Since the power of the input signal light is kept at a constant level in the front-end module 100, a monitor PD 103 for monitoring the input optical power is adopted in advance as a necessary component.
In a conventional front-end module, the monitor PD 103 is disposed outside of the front-end module, as illustrated in FIG. 1. In this front-end module, a fiber coupler with a 5% tap rate is provided on the input optical fiber to enable a monitor PD 103 external to the case 104 to monitor the power after splitting the input signal light.
However, with the configuration that provides a monitor PD external to the case as illustrated in FIG. 1, a large area on a board is required, making board mounting difficult. Thus, it is desirable to incorporate the monitor PD into the front-end module.
In response to such demand, there is the configuration illustrated in FIG. 2, in which a monitor PD 103 is built into the case of a front-end module 110. Mounted inside the case of the front-end module 110 are an optical signal processing circuit (dual polarization optical hybrid (DPOH)) 111 made up of a planar lightwave circuit (PLC), optical lenses 112 and 113, and an optoelectronic conversion processor (OE unit) 114 that includes optical semiconductors and electronic circuits. The DPOH 111 processes signal light and LO light respectively input from the two optical inputs 101 and 102. The optical lenses 112 and 113 condense the output light from the DPOH 111. The OE unit 114 optoelectronically converts the condensed light for output as an electrical signal. In the front-end module 110 illustrated in FIG. 2, an output port to the monitor PD 103 is provided on the same edge as the signal light output port of the DPOH 111. With a configuration providing a 5% tap circuit on the DPOH 111, part of the power from the input signal light is output to the monitor PD 103. Such an arrangement is easy given the circuit layout of the DPOH.